1. Technical Field
The present invention relates to a method for reducing metal artifacts in computed tomography (CT), and more particularly, to a method for reducing metal artifacts in CT, which restores an original image of an area generated by a metal in an X-ray sinogram by extracting pre-information from an image restored to a small size.
2. Description of the Related Art
Computed tomography (CT) provides 2-dimensional and 3-dimensional high-resolution tomographic images of a human body from a measured X-ray projection image.
Although there is a controversy over dangers related to exposure to X-ray emission, CT has been widely used for the purpose of diagnosis and treatment as a powerful tool for investigating an inside of the human body and as various medical imaging techniques for disease. A metal object transplanted into the human body can cause serious side effects in a CT image and can deteriorate quality of a reconstructed CT image.
Generally, metals strongly attenuate an X-ray beam and measured attenuation of the metal given X-ray beam on the detector suffer from severe photon starvation. Thus, after-image projection data becomes inaccurate.
Metal artifact reduction (MAR) for improving quality of the CT image is one issue of CT application in clinical practice, and several MAR algorithms have been proposed over the past 30 years. R. M. Lewitt and R. H. T. Bates developed the first MAR image reconstruction method using an incomplete projection image (R. M. Lewitt and R. H. T. Bates 1978 Image reconstruction from projections: IV: Projection completion methods (computational examples), Optik 50, pp. 269-278). Here, projection measurement through metal is presumed to be omitted and is restored by polynomial interpolation. In addition to linear and polynomial interpolation, wavelet interpolation, sinogram interpolation and normalized MAR interpolation techniques have been proposed to fill the omitted projection data. These MAR algorithms can be classified into projection and sinogram completion methods. Over the last 10 years, iterative methods of modeling a physical phenomenon of metal artifacts have established another class of MAR algorithm, wherein noise and beam hardening are modeled. As compared with the projection completion methods, model-based iterative methods are computationally intensive and have a limitation in clinical application.
In the projection completion method, omitted projections for a metal trace can be filled by seriating flow of undamaged projections adjoining the metal trace using various restoration algorithms such as interpolation and total variation. Such typical methods, in which gaps of the omitted projections for an area of the metal trace in a sinogram are filled with the undamaged projections, can distort an attenuation coefficient outside the metal object.
FIGS. 1(a), 1(b) and 1(c) show a phantom model including two metals placed in a white area, a sinogram thereof, and a sinogram area in which a projection passing through a red point corresponds to an angle range surrounded by a metal area, respectively. As the point approaches the metal area, the angle, range, in which the projection passing through the point is disturbed by the metal, becomes larger. Therefore, existing filling methods can cause inaccurate information on the projections passing through the red point. As a result, a CT image reconstructed from such a corrected sinogram by a filtered back projection (FBP) method can be less efficient in restoring details of a true image of the phantom model. Efficiency of the projection completion method has a high dependency on accuracy of synthesized data.
FIG. 1 (a) shows the phantom model including the two metals placed in the white area, and FIG. 1 (b) is the sinogram thereof. Blue and red curves in FIG. 1 (b) show traces of projections passing through blue and red points placed outside the metal area in FIG. 1 (a), respectively. FIG. 1 (c) shows a zoomed-in image of an area surrounded by a box in FIG. 1 (b). The projections passing through the red point are blocked in a projection angle range by the metal.